1. Technical Field
The present disclosure relates to a variable gain amplifier that varies signal feedback to an operational amplifier to control amplification of an output signal and an audio device incorporating the variable gain amplifier, and more particularly to a variable gain amplifier, incorporated in an audio device, to effectively reduce a popping noise when the gain is changed, and an audio device incorporating the variable gain amplifier.
2. Description of the Background Art
Amplifiers (variable gain amplifiers) that can vary gain to control and adjust volume level of audio signals are widely used in audio devices having a radio receiving function and a playback function to play back compact disks (CD) or mini disks (MD).
The variable gain amplifiers generally use operational amplifiers and feedback circuits, with the feedback circuit used to feed back an output signal output from an output terminal of the operational amplifier to an inverting input terminal of the operational amplifier and change the amount of feedback, which allows the variable gain amplifiers to vary their gain. Circuits for audio devices in which variable gain is used for control of sound amplification are proposed in JP-H09-093064-A and JP-H06-291573-A.
In a known variable gain amplifier, an attenuator to attenuate the signal to a suitable signal level is used as the feedback circuit, with resistance-dividing ratio in the feedback circuit changed by a digital control signal from, for example, a microcomputer.
FIG. 1 is a block diagram illustrating a configuration of a conventional variable gain amplifier 60. As illustrated in FIG. 1, the variable gain amplifier 60 includes an operational amplifier (OP) 61 and a feedback circuit (FB) 62 connected to the operational amplifier 61.
The feedback circuit 62 includes resistors and switching circuitry including multiple switches. The feedback circuit 62 adjusts an amount of voltage to be fed back to an inverting input terminal (−) of the operational amplifier 61 by opening and closing the switches in accordance with a control signal from a level control circuit to change a resistance-dividing ratio in the feedback circuit 62.
However, in this configuration, since the gain varies in steps, a noise called “popping” caused by rapid fluctuation in signal level of audio signal may be generated when the gain varies.
In order to prevent generation of the popping noise when the gain varies, improving resolution of gain settings and reducing the rapid fluctuation in the signal level are required.
As for a technique of improving the resolution of gain settings, it is easily conceivable that the number of resistors and switches is simply increased. However, in this technique, the size of circuit is increased, which is impractical.
Alternatively, as illustrated in FIG. 2, a configuration in which two variable gain amplifiers are provided and both are connected in series is known.
However, in the circuit shown in FIG. 2, two operational amplifiers are required in the two variable gain amplifiers 101 and 102 and an output noise level of the audio signal is increased by connecting the two operational amplifiers in series.
As illustrated in FIG. 3, a configuration in which the feedback circuit 62 is constituted by a D-A multiplier using R-2R digital-analog converter (DAC) consisting of a ladder network of resistors with only two values is proposed as yet another known technique.
However, in the D-A multiplier circuit including the R-2R DAC shown in FIG. 3, due to the nature of the circuit, on resistances of the switches in the switching circuit may causes a distortion rate of the audio signal to degrade. That is, when the R-2R DAC of the D-A multiplier in a variable gain amplifier is used in an audio playback device, since resistances of the resistors in the R-2R DAC cannot be made very large, it is difficult to obtain the switches whose on resistances are sufficiently small with respect to the resistors in the R-2R DAC, thereby inevitably causing the distortion rate to deteriorate.
In addition, differing from the configuration in which the variable gain amplifier that varies the feedback amount of the output signal of the operational amplifier fed back to the inverting input terminal thereof, a sound signal processor is proposed in JP-2008-109560-A to prevent generation of the popping noise. The sound signal processor handles soft mute to alleviate the noise generated when the mode is changed and a power supply is on or off. In this sound signal processor, a mute time is set to a time around half the wavelength of a minimum frequency in a playback band, which prevents generation of the popping noise when the mute is on or off.
In addition, an audio circuit that feeds back an output signal of the operational amplifier to the inverting input terminal thereof and does not vary the feedback amount is proposed in JP-2010-021175-A. In this configuration, the audio circuit outputs a first reference voltage to be input to the non-inverting input terminal of the operational amplifier that is raised in a state in which the voltage is half a normal voltage and the waveform is symmetrical above and below the normal voltage when the power is on, and when the power is off outputs a second reference voltage whose voltage is symmetrical to the voltage while the power is on. Therefore, an unnecessary high-frequency wave component is reduced, which alleviates the popping noise. However, in this configuration, complicated circuitry is required for generating the reference voltages.